This invention relates to vehicle steering systems and more particularly to a device for holding the steerable wheels of a vehicle, such as a motor home, bus, truck, automobile or the like, so that a center steering position is maintained in spite of spurious steering inputs, such as those caused by variable crosswinds, crown curvature or slant of the highway, or other factors tending to adversely affect vehicle steering by the driver.
The steering systems of highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
Vehicles with steering systems having positive caster generally track relatively straight ahead and generally resist normal steering inputs away from center, including those of the driver. Intentional turning maneuvers by the driver therefore require sufficient turning force to overcome this positive resistance to movement away from center. When the driver relaxes the turning force applied to the steering wheel, a positive caster system has a definite tendency to return to its straight ahead position, although it may overshoot the neutral or center position if the steering wheel is entirely released.
While positive caster is desirable in some respects, it is not without compromises over the full steering spectrum. For example, the adverse effects of strong gusty cross winds are usually more pronounced with large amounts of positive caster. As its name would imply, the vehicle tends to caster towards the side of the roadway to which it is being pushed by the wind. Similarly, a high crown at the center of the roadway or a slanted roadway can cause vehicles to turn toward the edge of the roadway, that is, in the downhill direction. In addition, generous positive caster provides significant resistance to small radius turns, which can make city driving quite fatiguing. These three adverse effects are some of the negative aspects of achieving steering stability through generous amounts of positive caster.
Another drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly before encountering any stabilizing resistance from the steering wheel. The negative action of the steerable wheels is caused by spurious steering inputs from crosswinds, slanted or crown roads, bad road surfaces, and other adverse dynamic steering forces. Inherent geometric steering characteristics may also be responsible for spurious steering inputs.
The transmission of these various inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore produce driver fatigue.
The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver""s steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.
The invention provides improved on-center holding of the steerable wheels, and significantly reduces driver fatigue because it results in a major reduction in driver steering inputs. The holding assembly is easily activated by the driver while driving the vehicle, and its activation makes driving more pleasurable and less fatiguing.
The center holding assembly of the invention comprises linkage means of variable length that extends between the steerable wheels and an axle or frame member such that the length of the linkage means defines the center position of the steering system. The linkage means comprises a resistance assembly that provides a resistance force for resisting steering forces tending to move the steerable wheels to either side of the center position, and a center holding assembly for transmitting the steering forces to the resistance assembly.
The holding assembly comprises a holding piston, a holding cylinder providing first and second holding chambers, one on each side of the holding piston, a fluid transfer system for providing a flow of fluid to and from each of the holding chambers, and a solenoid operated valve for controlling the fluid flow. The control valve is operable between a closed position for preventing the fluid flow to hold the piston in a locked center position, and an open position for allowing the piston to move away from its center position in the holding cylinder. Movement of the holding piston causes fluid flow to one of the holding chambers and fluid flow from the other of the holding chambers. This fluid flow permits the length of the linkage assembly to change relatively freely in response to steering forces, which in turn permits the steerable wheels to move freely to either side of their center position in response to applied steering forces for small radius turns as used for going around corners in city driving.
The resistance force provided by the resistance assembly is preferably produced either by a dual spring mechanism or by a second fluid system comprising a pair of pistons in a cylinder, and an accumulator. An electrical actuator system is responsive to the resistance force for operating the solenoid control valve so as to prevent holding piston movement away from its locked center position until the resistance force reaches a predetermined level.
The actuator system includes a first sensor for providing a first signal in response to the predetermined resistance force, a second sensor for providing a second signal in response to the holding piston being in its locked position, and a control assembly. The control assembly causes the solenoid valve to move to its open position in response to the first signal and thereafter keeps the valve open during holding piston movement away from its locked position. The control assembly actuates the solenoid valve to its closed position only when the holding piston returns to its locked position after the vehicle has completed a small radius turn.
The resistance assembly includes a component that moves in response to the applied steering force, and movement of this component resists relatively small and limited changes in the length of the linkage assembly, such that relatively small movements of the steerable wheels to either side of their center position are opposed by the resistance force. These small movements correspond to the very large radius turns that occur when a vehicle is steered through maneuvers at highway speeds (as opposed to the small radius turns that occur when a vehicle turns a corner). Thus, during large radius turns, the resistance assembly provides a resistance force that biases the steerable wheels back toward their center position, and this bias serves as a return force to return the steerable wheels to their center position upon removal of the steering force producing the large radius turn.
On the other hand, during small radius turns, the resistance assembly is rendered ineffective (and may be said to be deactivated) by the control assembly causing opening of the solenoid valve, which in turn allows movement of the holding piston away from its locked position in response to the steering force applied to produce the small radius turn. A preferred way of xe2x80x9ctriggeringxe2x80x9d the opening of the solenoid valve is for the first sensor to provide its signal in response to a predetermined amount of the aforesaid resistance component movement that is proportional to the resistance force.
Once the solenoid valve is opened to allow movement of the holding piston away from its locked center position, this valve remains open until the piston returns to its center position so that no resistance force will be applied to the steering system during small radius turning maneuvers of the steerable wheels. In addition, the actuator system includes a remote switch for turning the actuator system on and off remotely from the driver""s station of a vehicle. When the remote switch is in its off position, the solenoid valve remains open so that no resistance force is applied to the steering system by the linkage assembly because the holding piston is not xe2x80x9clockedxe2x80x9d and is therefore free to move away from its center position in the holding cylinder.
The assembly is preferably connected between the steering system and the front axle or a nearby frame member of the vehicle in a position that allows the steerable member(s) to move through its full range of steering movements while providing sufficient leverage for the apparatus to resist movement of the steerable member away from the center position producing straight ahead travel of the vehicle. The steering system connection may be made to any steering system component providing appropriate range and leverage, such as a tie rod which joins the two front steerable wheels of a highway vehicle, or the pitman arm connected to the reduction gear. The frame connection may be made to any component serving as a fixed mounting relative to the steering system. This fixed component may be a frame member, or an axle or some other part carried by the vehicle frame instead of an actual frame member.
The invention may be used with steering systems having a reduction gear between the steering wheel and the steerable wheels. In this application, the assembly is preferably connected to the steering system at a location between the steerable wheels and the reduction gear so as to be unaffected by any slack in the reduction gear or in components and connections between the reduction gear and the steering wheel. It is therefore on the slow side of the reduction gear ratio. The invention thus provides a zero backlash center holding assembly.
An adjustment means accessible by a mechanic provides for different levels of steering force to initiate or breakaway into a steering movement away from center, depending on the size, type and steering characteristics of the vehicle. This level of force is sometimes referred to in this specification as the xe2x80x9cbreak away resistancexe2x80x9d. Different levels of break away resistance and of resistance force may also be appropriate to compensate for changes in the forces acting upon the vehicle. Thus, the resistance force provided by the invention may be increased or decreased to provide a level of force sufficient to overcome any spurious steering inputs and to suit driver road feel, particularly a feel of the steering wheel that lets the driver know when the steered wheels are beginning to move away from center and are closely approaching return to center.
In the absence of the invention, spurious inputs to and/or mechanical slack in the steering assembly require almost constant manipulation of the steering wheel by the driver and make it almost impossible for the driver to hold the vehicle on a true straight ahead course. Use of the invention therefore permits a substantial reduction in the caster angle of vehicles with positive caster, thereby significantly reducing the crosswind effect and provides the driver with a positive touch control not heretofore attainable with positive caster.
In other words, the invention provides a distinctive feel when approaching or leaving the center position. Thus, the sense of touch is added to the visual sense to aid control of the vehicle and reduce driver fatigue. The turning resistance selected should satisfy the road feel desired by the driver and be sufficient to overcome anticipated spurious inputs. Positive stability is thereby achieved for previously unstable steering systems.
Although the present invention is particularly useful as a center holding assembly for motor vehicles, it can be employed to hold the center position of any steerable member moveable to either side of a preselected position. For example, the assembly can keep an outboard motor centered so that a boat follows a straight course over the water in the presence of spurious steering forces produced by wind and wave action. The assembly can also be used to keep centered such steerable members as the rudders of ships or airplanes and the tongues of tandem trailers or railway cars. The assembly is useable with both power and non-powered steering systems, with the level of holding forces provided usually being less for vehicles without power steering.